There are many catalysts that are known for the conversion of syngas to methanol. However, thermodynamic considerations limit the syngas conversion per pass to the 20-30% range for methanol production. Thus, substantial amounts of costly syngas recycled are required to produce methanol. On the other hand, such thermodynamic limits do not exist when converting syngas to dimethyl ehter, and, hence, very deep conversions are possible when preparing dimethyl ether. Dimethyl ether is a very useful chemical intermediate for the conversion to other useful products. In principle then, dimethyl ether could be a more attractive intermediate from a cost input than is methanol for the production of other products.
U.S. Pat. No. 4,098,809, issued July 4, 1978, generally discloses the use of a copper/zinc/chromium catalyst combined with alumina for the conversion of a mixture of CO, CO.sub.2 and H.sub.2, wherein the quantity of CO is in excess of the stoichiometric value, to dimethyl ether.
Application Ser. No. 313,420, filed Oct. 21, 1981, discloses the use of a copper/zinc catalyst supported on alumina having a sodium concentration of less than 700 ppm is useful for converting syngas to dimethyl ether in high yield. It is also known that the use of certain alkali metal oxidic compounds such as, for example, potassium carbonate or potassium oxide, as promotors of catalysts utilized in hydrocarbon processes, extends the life of these catalysts. The addition of alkali metals, however, to catalysts such as a copper-zinc-chromium/alumina catalyst of U.S. Pat. No. 4,098,809 or the copper-zinc/alumina catalysts of Ser. No. 313,420, kills the selectivity of these catalysts to dimethyl ether and causes them to produce primarily methanol. It has been found that alkali metal dopants can be utilized for a copper-zinc alumina catalyst, while at the same time maintaining selectivity to the dimethyl ether, by utilizing a physical mixture of two catalyst components; a first component comprising said alkali metal doped copper-zinc/alumina catalysts, and a second component comprising WO.sub.3 /silica-alumina component. It has been noted that the addition of WO.sub.3 to silica-aluminas used as catalysts in hydrocarbon processes provides for longer life thereof.